1. Field of the Invention
The present invention relates to refrigerators having a quick freezing function.
2. Description of the Prior Art
With greater attention directed to home freezing in recent years owing to diversified dietary habits, it has become desirable for household refrigerators to have a quick freezing function.
Common fan-type freezer-refrigerators include a loop of compressor, a condenser, a capillary tube and cooler, but include no specific expedient in its cycle for quick freezing. When such a refrigerator is used for this purpose, the compressor is continuously operated for a given period of time disregarding the signal of the thermostat for detecting the temperature of the freezer compartment to lower the temperature of the freezer compartment, i.e. the temperature of cold air forced out through the cooler, to the greatest possible extent and thereby expedite freezing.
During quick freezing, however, the cycle itself operates in the usual mode, with the compressor merely held in continuous operation, so that there is a limitation to the reduction of temperature of the cooler or freezer compartment. Moreover, freezing is effected primarily with cold air, therefore involves a small heat transfer coefficient and cannot be expedited greatly as expected. Accordingly, several refrigeration cycles have been proposed which are adapted for quick freezing.
For example, Unexamined Japanese Patent Publication SHO 55-121369 discloses a refrigeration circuit which includes a first capillary tube, a first cooler, a series circuit of second capillary tube and second cooler provided therebetween, and an electromagnetic valve connected in parallel with the series circuit. The valve is opened for usual operation or is closed for quick freezing. More specifically, when the electromagnetic valve is open, the refrigerant almost wholly flows through the valve owing to the resistance of the second capillary tube without allowing the second cooler to perform a substantial cooling function, whereas when the valve is closed, the refrigerant flows through the first and second capillary tubes and is thereby throttled in two steps, causing the second cooler to function along with the first to give a lower temperature. The second cooler is generally in the form of a direct cooling plate of metal, and food is placed directly on the plate and thereby frozen quickly through improved heat transfer. The refrigerant flowing out of the condenser is throttled in two steps by the first and second capillary tubes for rapid freezing and consequently affords a lower temperature on evaporation than in the usual operation. Nevertheless, since the refrigerant passes through both the second and first coolers, the evaporators combined have a larger area of heat transfer, which makes it impossible to obtain an extremely low temperature.
Unexamined Japanese Patent Publication SHO 56-37475 proposes a cycle which comprises a first capillary tube, a second cooler, a parallel circuit of second capillary tube and first cooler provided therebetween, and an electromagnetic valve connected in series with the first cooler. The valve is opened for usual operation or is closed for quick freezing. When the valve is open, the refrigerant from the first capillary tube flows through the first and second coolers, while with the valve closed, the refrigerant is throttled in two steps by the first and second capillary tubes and then evaporates only at the second cooler, which therefore gives a temperature as low as about -50.degree. C. Thus, the cycle is useful for quick freezing. However, the refrigerant flows through both the first and second coolers during usual operation, so that the second cooler also becomes frosted which is generally in the form of a direct cooling plate installed in the freezer compartment. Accordingly, the cycle has the drawback that the second cooler can not be automatically defrosted but must be manually defrosted after removing the contents of the compartment therefrom.